1. Field of the Invention
The present invention relates to an exhaust gas recirculation valve for use in an exhaust gas recirculation system such as an internal combustion engine for vehicles or the like.
2. Description of the Prior Art
In general, in order to reduce the amount of nitrogen oxides (NOx) included in exhaust gas from an internal combustion engine disposed in an automobile or the like, an exhaust gas recirculation control system has been used for feeding a part of the exhaust gas back into the inlet of the internal combustion engine so as to burn the part of the exhaust gas again.
Referring now to FIG. 16, it illustrates an exhaust gas recirculation system incorporating a prior art exhaust gas recirculation valve abbreviated as an EGR valve. In the figure, reference numeral 1 denotes an internal combustion engine, 2 denotes a combustion chamber disposed in the internal combustion engine 1, 3 denotes an exhaust pipe connected with the combustion chamber 2 for making exhaust gas passing therethrough, 4 denotes an exhaust gas recirculation cooler or an EGR cooler connected with the exhaust pipe 3 for taking in a part of the exhaust gas from the combustion chamber 2 so as to cool it, 5 denotes an exhaust gas recirculation valve (or EGR valve) connected with the EGR cooler 4 for receiving the exhaust gas cooled by the EGR cooler 4 and for adjusting the amount of the exhaust gas to be recirculated into the inlet pipe (not shown in the figure) of the internal combustion engine 1, 10 denotes a solenoid valve, to which a duty signal, in which the transition between its on state and its off state takes place tens times per second, having a duty cycle according to the load on the engine is applied, for supplying compressed air at a pressure according to the duty cycle to the exhaust gas recirculation valve 5, and 11 denotes a pressure reducing valve for reducing the pressure of the compressed air from a compressor mounted in an automobile or the like to a predetermined pressure and for supplying the air at the predetermined pressure to the solenoid valve 10.
The EGR valve 5 is provided with a control valve 6 and a pressure chamber 9 separated from another space in the EGR valve by a sealing member 7 and a piston ring 8. Air at a positive pressure is applied to the pressure chamber 9 from a compressor (not shown in the figure) mounted in the automobile by way of the pressure reducing valve 11 and solenoid valve 10 which controls the pressure of the compressed air passing through the pressure reducing valve 11 in response to the duty signal. The exhaust gas recirculation valve 5 is so constructed as to change the opening of the control valve 6 thereof according to the pressure of the air fed by the solenoid valve 10 in order to control the amount of the exhaust gas to be recirculated. The exhaust gas recirculated under the control of the EGR valve 5 is fed into the inlet pipe (not shown in the figure) of the internal combustion engine 1 and is then mixed with air sucked into the engine, with the result that the temperature in the combustion chamber 2 of the internal combustion engine 1 decreases and hence the amount of nitrogen oxides included in exhaust gas is reduced.
The exhaust gas is kept at a high temperature just after it is discharged out of the exhaust pipe 3, though its temperature decreases by the time it reaches the exhaust gas recirculation valve 5 because it is cooled by the EGR cooler 4. As a result, the amount of heat transferred from the exhaust gas to the EGR valve 5 is reduced and hence the degree of aging of the sealing member 7 and piston ring 8 due to the heat is reduced.
Referring now to FIG. 17, it illustrates a cross-sectional view showing the structure of the aforementioned prior art EGR valve 5. In the figure, reference numeral 12 denotes a housing having an exhaust gas path 15 disposed therein and an air inlet 12a, 13 denotes an exhaust gas inlet of the path 15 for receiving the exhaust gas discharged out of the exhaust pipe 3 of the internal combustion engine 1, 14 denotes an exhaust gas outlet of the path 15 for feeding the exhaust gas into the inlet pipe (not shown in the figure) of the internal combustion engine 1, and 16 denotes a doughnut-shaped valve seat disposed in the exhaust gas path 13 inside the housing 12. The control valve 6 can come into contact with the valve seat 16. Furthermore, reference numeral 17 denotes a sliding member mounted in the housing 12 and slidably engaged with the control valve 16 which can slide up and down, 17a denotes a cylindrical holder disposed under the sliding member 17 mounted in the housing 12, and located in the upper portion of the exhaust gas path 15 for preventing carbon or the like included in the exhaust gas from entering the EGR valve 5, 17b denotes a metallic fiber filter built in the holder 17a for cutting down carbon or the like adhered to the sliding member of the control valve 6 to prevent the carbon or the like included in the exhaust gas from entering the EGR valve 5.
Reference numeral 18 denotes a disc-shaped pressure plate, the center portion of which is secured to the upper end of the control valve 6 with a nut 18a, 19 denotes a compressed spring for pushing the pressure plate 18 in an Upward direction, 20 denotes a cylinder secured to the housing 12 with bolts 20a, and 22 denotes a piston which can slide within the cylinder 20. The sealing member 7 for sealing and forming the pressure chamber 9 and the piston ring 8 for ensuring that the piston 22 slides stably within the cylinder 20 are located in the space between the cylinder 20 and the piston 22. A pressure applying port 21 for feeding control air at a predetermined pressure into the pressure chamber 9 is attached to the cylinder 20.
In the prior art exhaust gas recirculation valve 5 having the structure mentioned above, the piston 22 is pushed downward in such a manner that its travel depends on the pressure of the control air fed into the pressure chamber 9 through the pressure applying port 21, and the control valve 6 is opened in such a manner that its opening depends on the travel of the piston, with the result that the exhaust gas discharged out of the exhaust pipe 3 of the internal combustion engine 1 enters the exhaust gas path 15 by way of the inlet 13 of the path and reaches the inlet of the engine by way of the outlet 14 of the path. Then, the exhaust gas is mixed with a mixture of fuel and air and is then fed into the combustion chamber 2 of the internal combustion engine 1 for the afterburning of the exhaust gas. Thus, the amount of nitrogen oxides (NOx) which is a toxic constituent included in the exhaust gas is reduced.
Air from a compressor for use in an automobile is kept at high pressure; its pressure is typically between 5 kg/cm.sup.2 and 9 kg/cm.sup.2. Therefore, when the compressed air from the compressor is directly applied to the solenoid valve 10 in which the switching between on and off is repeated tens times per second, it is difficult to control the opening of the control valve 6 of the exhaust gas recirculation valve 5 to within a range of fine opening levels or middle opening levels. Therefore, after the pressure of the compressed air is reduced to a predetermined pressure by the pressure reducing valve 11, it is supplied to the solenoid valve 10.
In the prior art exhaust gas recirculation valve having the aforementioned structure, the middle opening controlling operation is performed with control of the duty cycle of a signal applied to the solenoid valve 10, though the pressure of the air supplied to the pressure chamber 9 by way of the solenoid valve 10 varies because the pressure of the air passing through the pressure reducing valve 11 varies with a change in the pressure of the compressed air from the compressor as an air source. Therefore, the prior art exhaust gas recirculation valve suffers from a disadvantage that the exhaust gas recirculation cannot be performed effectively because of changes in the opening of the control valve 6 within a range of fine opening levels, e.g., in the 40% to 20% opening level range, or within a range of middle opening levels, which must be controlled with high accuracy from the viewpoint of the reduction of NOx in the internal combustion engine and the increase of gas mileage.
Another disadvantage is that it is difficult to stably maintain the opening of the control valve 6 at a fine opening level because the pressure of the air supplied to the pressure chamber 9 by the solenoid valve 10 must be limited when controlling the opening of the control valve to within a range of middle opening levels, and endurance of the exhaust gas recirculation valve 5 is reduced remarkably because a collision between the control valve 6, the opening of which is maintained at a small opening level, and the valve seat 16 is repeated.